The inherent strength and beauty of natural wood is easily compromised upon exposure to the environment due to poor surface stability and weatherability. A polymer coating imparts two important functions to the underlying wood substrate: the aesthetic function enhances the appearance of the natural wood grain while the protective function preserves these aesthetics by preventing damage from solvents, moisture, heat, decay and mechanical sources. Properly protected and maintained wood can last for a long time, but finished wood typically must endure periodic refinishing which can be costly and damaging to the underlying wood. Thus, maximizing the durability of the protective finish is paramount to providing optimal protection. Due to routine wear and tear, surface scratches are generated which develop micro-cracks, eventually leading to macroscopic damage that causes the coating to lose its aesthetic and protective functions. The use of a self-healing coating will increase the operational life of coatings and eliminate the need to frequently refinish damaged coatings. In addition to tremendous cost savings, this results in significant energy savings from fabrication and application of the coating materials and conserves supplies of raw wood, an important natural resource.
When a polymer coating is mechanically damaged, it is either plastically deformed at the surface (such as a blunt indent or a light scratch), or cracked (such as a cut made with a sharp blade). The majority of existing self-healing technologies do not target both forms of damage. One is neglected at the expense of the other. An example is the polyurethane based self-healing coating developed by Bayer Material Science for the automotive industry [van Benthem, R.; Ming, W.; de With, G., “Self Healing Polymer Coatings,” In Self Healing Materials, 2007, pp 139-159; http://www.bayercoatings.com/BMS/DBRSC/BMS_RSC_CAS.nsf/id/COEN—7EAClearcoat]. They utilized the shape memory effect (termed as “reflow effect”) to recover plastic deformation, for example the scratch from a car wash. There is no “self-healing” in the case of cracking or micro-cracking due to the lack of chemical or physical forces to “re-bond” the newly generated crack surfaces. On the other hand, Cho et al. [Cho, S. H.; White, S. R.; Braun, P. V., “Self-Healing Polymer Coatings,” Advanced Materials, 21, (6), 645-649, (2009).] developed a coating containing encapsulated healing agents and catalysts that specifically tackles crack healing/re-bonding, based on their previous success with bulk self-healing polymers using the same strategy [White, S. R.; Sottos, N. R.; Geubelle, P. H.; Moore, J. S.; Kessler, M. R.; Sriram, S. R.; Brown, E. N.; Viswanathan, S., “Autonomic healing of polymer composites,” Nature, 409, (6822), 794-797, (2001).]. A surface crack made by hand scribing with a sharp razor blade ruptures the capsules and triggers the crosslinking of the released healing agent to seal the crack. This strategy is only applicable to healing cracks, since the formation of a crack is necessary to rupture the capsules where no capsules are likely to be ruptured (therefore no healing agent released) during plastic deformation. Besides, the large size of the microcapsules (50 μm and above) limits the use of this technology in many self-healing coatings. A similar methodology uses hollow fibers [Dry, C., Comp. Struc., “Procedures developed for self-repair of polymer matrix composite materials,” 35, 263-269 (1996).] or interconnected microchannels [Toohey, K. S.; White, S. R.; Sottos, N. R., “Self-healing polymer coatings,” Proc. 2005 SEM annual conference and exposition on experimental and applied mechanics, 2005, p 241-244.] to store the healing materials. Fabrication of microvascular network is a challenge, which limits its application. Besides polymerization based healing, reversible chemistry based on Diels-Alder reaction [Chen, X. X.; Dam, M. A.; Ono, K.; Mal, A.; Shen, H. B.; Nutt, S. R.; Sheran, K.; Wudl, F., “A thermally re-mendable cross-linked polymeric material,” Science, 295, 1698-1702, (2002).], hydrogen bonded supramolecular network [Cordier, P.; Tournilhac, F.; Soulie-Ziakovic, C.; Leibler, L. Nature, 451, 977-980, (2008).], and ionic liquids [Long, T., “Ionic Liquid and Ion-Containing Polymers for Shape Memory and Self-healing.” Lecture at Smart Coatings 2009.] has been utilized for producing healable polymer materials. All of these are limited to a narrow set of less commonly used polymers, rendering widespread utilization of the technology unlikely. A unique approach among these methods is a thermoset/thermoplastic mixture that offers crack sealing capacity when the material is heated, and the thermoplastic is able to diffuse across the crack boundary [Jones, F. and Hayes, S. A., “Self Healing Composite Material”, WO 2005/066244 A2.]. However, in this system, the thermoset and thermoplastic are miscible, e.g., they form a single phase. When two polymers having different glass transition temperatures (Tg) are blended into a single phase material, a single glass transition temperature exists, which takes a value in between the glass transition temperatures of the two composing polymers. This means that adding a miscible thermal plastic to the thermoset lowers its glass transition temperature, and hence the mechanical performance such as hardness. Most recently, researchers from University of Southern Mississippi invented a polyurethane based self-healing material by mixing chitosan into the compound [http://www.msnbc.msn.com/id/29663741/from/ET/.]. Upon exposure to UV light, chitosan rods are broken up and bond to each other across the damaged area. A drawback of this technology is that repeated healing is not possible.
Bayer Material Sciences, supplier of polyurethane resins for acrylic-polyurethane and all-polyurethane clearcoats, developed self-healing polyurethane automobile topcoats that self heal small scratches. The self-healing mechanism is based on physical deformation of a thermoset polymer network. When a small physical deformation occurs on a clear automobile topcoat such as that induced by a car wash, integrity of the coating is compromised. When a gentle heat treatment such as a hot afternoon sunshine brings the surface temperature to above the glass transition temperature of the polymer, the physically deformed network restores to its original shape and therefore heals the minor surface scratches. The coating will not self-heal more severe damages such as incurred by a car key scratch where the polymer network is broken.